1. Field of the Invention
An object of the present invention is a phase demodulator of the DPSK (differential phase shift keying) type.
Another object of the invention is the application of this device to a landing aid system of the MLS (microwave landing system) type.
2. Description of the Prior Art
Increasing use is being made, in data transmission, of the modulation of the phase of a radioelectrical carrier wave. In general the signal is written as: EQU s(t)=A.multidot.Sin [.omega..sub.o t+.theta.(t) (1)
where A is the amplitude of this signal, .omega..sub.O is the pulsation that corresponds to the carrier frequency and O(t), is the modulation frequency as a function of time. For biphase shift keying (BPSK) modulation, the signal .theta.(t) will be equal alternately to 0 or to .pi. depending on whether the bit sent is 0 or 1. This type of modulation can be extended to multiple-phase shift keying (MPSK) modulation.
At reception, it is therefore necessary to demodulate (or decode) the signal received to extract the useful signal .theta.(t) in the composite signal s(t). The demodulator according to the invention aims at achieving a demodulation of this type when neither the frequency nor the phase of the carrier wave is known with precision.
To resolve this problem, various approaches are known and are especially described in Spilker Jr., Digital Communications by Satellite, Prentice Hall. One of these approaches, described on page 304 and called the Costa loop, uses a voltage controlled oscillator (VCO) and an automatic control loop. Another approach, described on page 302, also uses a VCO type oscillator and automatic control loops, It further comprises frequency doubling devices. The main drawback of these devices is their complicated nature and the fact that they require a VCO-type oscillator, which makes them expensive.
An object of the present invention is a phase demodulator which, in particular, does not require any oscillator of the VCO type.
To this end, it uses an oscillator feeding a phase demodulator. The output signal of the phase demodulator is mixed with the demodulated signal and is applied to a loop, the output signal of which:
controls the demodulator;
constitutes the output signal of the demodulator.